The present invention relates to optical waveguide filters for performing arbitrary filtering operations useful in optical communication, optical exchange, or other optical signal processing applications. Specifically, this invention relates to tunable filters by which the filtering operation can be modified to accommodate different filtering response functions, all using the same physical device. By optimizing the lengths of waveguides in the filter, unprecedented filtering performance may be achieved.
Optical waveguide filters are important devices in optical communication and optical signal processing and can perform many useful functions of optical filtering. For example, in optical fiber communication, signals experience chromatic dispersion, which spreads each optical pulse among neighboring pulses. This spreading renders the signal unintelligible to the receiver. However, an optical filter, whose response inverts the chromatic dispersion, can recompress the pulses and restore the signal fidelity. Such a filter is called a chromatic dispersion compensator (CDC).
For CDC applications, several different filtering response functions may be required of the same device. The amount of chromatic dispersion experienced by a signal depends on the length and type of fiber through which the signal propagates. If the routing of a signal should change, the length/type of fiber through which the signal propagates will change. This causes the chromatic dispersion to change. Hence, it is advantageous for a CDC device to be tunable so that several different lengths of fiber can be compensated using the same device. If the CDC device is not tunable, it must be replaced whenever the signal routing (and therefore dispersion) changes, which is costly and cumbersome. Tunable optical filters can therefore have a real advantage over nontunable filters.
Optical waveguide filters are particularly suitable for practical applications because they may be implemented monolithically on substrates, such as silica-on-silicon and polymer-based monolithic waveguide technology. This facilitates manufacturability and long term stability.
There are two typical waveguide processing elements which may be used as building blocks to form optical filters. The first, shown in FIG. 1A, is the unequal arm Mach-Zehnder interferometer, and the second, shown in FIG. 1B, is the ring resonator. Such processing elements may be interconnected and/or combined in various ways to form more complex filters to achieve desired filtering functions. The Mach-Zehnder interferometer has a pair of waveguides, a first coupler between the two waveguides, a second coupler between the two waveguides, and a differential length, denoted Δlmz, of the waveguides between the two couplers. The ring resonator has a ring waveguide, a feeder waveguide, and a coupler providing coupling between the ring and feeder waveguides. The circumference of the ring is denoted Δlr.
To provide tunability of the Mach-Zehnder interferometer, a phase shifter may be used in one of the arms. To provide additional tunability, a variable, instead of fixed, coupler may be employed. (The behavior of fixed and tunable couplers is described below). A fully tunable Mach-Zehnder interferometer is illustrated in FIG. 2A. A partially tunable Mach-Zehnder, using a phase shifter and a fixed coupler, is shown in FIG. 2B. To provide tunability of the ring resonator, a phase shifter is used within the ring. The feeder wave guide is coupled with a tunable or fixed coupler to the ring waveguide, as shown in FIG. 2C and FIG. 2D.
A fixed coupler has a transfer matrix between its two input and two output waveguides given by:       H    ⁡          (      θ      )        =      (                                        cos            ⁢                                                   ⁢            θ                                                j            ⁢                                                   ⁢            sin            ⁢                                                   ⁢            θ                                                            js            ⁢                                                   ⁢            in            ⁢                                                   ⁢            θ                                                cos            ⁢                                                   ⁢            θ                                )  where the coupling angle θ is given by θ=2πΔnefflc/λ. The quantity Δneff is the effective index difference between the odd and even modes, lc is the coupling length, and λ is the light wavelength.
A variable coupler may be made from two fixed 90° couplers and a phase shifter, as represented in FIG. 3. Tuning of the couplers and phase shifter is accomplished with different methods. One method is to heat the waveguide, another is to induce stress into the waveguide with piezoelectric actuators, and still another is to place the waveguide under the influence of electrical fields.
Many different types of filter structures may be constructed from various combinations of the Mach-Zehnder interferometer and the ring resonator. For example, lattice optical filters comprised of a serial cascade of Mach-Zehnder elements (the same arrangement as illustrated in FIG. 4) have been described in U.S. Pat. No. 5,572,611; a paper by K. Jinguji and M. Kawachi, “Synthesis of Coherent Two-Port Lattice-Form Optical Delay-Line Circuit,” Journal of Lightwave Technology, Vol 13, No 1, January 1995, pp.73-81; and a text, Optical Filter Design and Analysis by Christi Madsen and Jian Zhao, John Wiley and Sons, 1999. Each element has the same differential delay. In these references, it is shown that the lattice structure is mathematically equivalent to a finite impulse response (FIR) filter. By employing the fully tunable Mach-Zehnder element in FIG. 2A, a single device may be tuned to achieve an arbitrary FIR filter response. Many other structures are possible, such as the all pass filter structure, made of a serial or parallel cascade of ring resonators. The Madsen and Zhao text provides a good overview of the different types of structures possible.
The performance and behavior of an optical waveguide filter are greatly affected by the choice of the lengths, Δlmz and/or Δlr (for filters using Mach-Zehnder and/or ring resonator processing elements in various combinations). An early example described in the Jinguji and Kawachi paper cited above, has a serial cascade of Mach-Zehnder stages, each with the same differential length. The frequency response of this filter is periodic, with free spectral range given by c/(Δlmzneff), where c is the speed of light, and neff is the effective index of the waveguide. As mentioned before, this filter is equivalent to a discrete-time FIR filter. U.S. Pat. No. 5,596,661 describes another serial cascade of Mach-Zehnder stages in which the differential lengths Δlmz in the stages are optimized for separating 1.3 and 1.55 μm telecommunications channels, and for flattening the gain of EDFAs (Erbium-Doped Fiber Amplifiers). The length optimization accounts for the nonideal behavior of couplers and waveguides. Additionally, fewer stages are needed to achieve the desired filtering than compared to lattice Mach-Zehnder filters with Δlmz equal in each stage. Tunability was not provided in this design.
For tunable filters, a paper by C. Madsen, G. Lenz, A. J. Bruce, M. A. Cappuzzo, L. T. Gomez, and R. E. Scotti, “Integrated All-Pass Filters for Tunable Dispersion and Dispersion Slope Compensation,” IEEE Photonics Letters, December 1999, pp. 1623-1625, describes the achievement of tunable dispersion compensation using cascaded ring resonator structures with actuators to tune the coupling from the feeder waveguide to the ring resonator. The amount of chromatic dispersion compensation may be tuned. The ring circumferences Δlr of the rings are selected according to a ‘Vernier’ design, in which the circumferences are all small multiples of a fundamental length, by which the free spectral range of the filter device is increased to the reciprocal of the fundamental length, rather than just the reciprocal of the circumference of one of the rings. U.S. Pat. No. 6,285,810 describes another optical filter in the form of a tunable add/drop multiplexer (ADM) useful in dense wavelength division multiplexing (DWDM) systems for injecting or extracting optical carriers of selected wavelengths, from a plurality of optical carriers of distinct wavelengths. In this optical filter, one or two of the waveguide lengths are optimized in a lattice filter similar to that of FIG. 4. All the other remaining waveguide lengths are equal, except these two, by which some improvement in performance is achieved.
The present invention recognizes that advancement can proceed even further. The filter described by K. Jinguji and M. Kawachi requires more stages in comparison to filters with better choices of lengths Δlmz and/or Δlr for better performance. Similarly, the nontunable filter of U.S. Pat. No. 5,596,661 is limited; only one filtering operation may be implemented. Even the described tunable filter devices have shortcomings. By constraining the circumferences ring resonators to be small multiples of a fundamental length, the filter of the Madsen et al. paper foregoes many useful capabilities. These capabilities include the simultaneous tunability of the dispersion slope, tunability of higher order dispersion, or tunability of the gain vs. frequency response. Similarly, the filter described in U.S. Pat. No. 6,285,810 requires more stages than necessary to achieve desired performance. Savings in the number of stages of 20-30% can be realized.
In accordance with the present invention, a fully tunable optical filter is provided which addresses the requirements of performance and flexibility.